A dense wavelength division multiplexing-passive optical network (DWDM-PON) is widely regarded as an ultimate optical network of a next generation. The most important subject to consider with regard to wavelength division multiplexing-passive optical network (WDM-PON) technology is that, although a plurality of optical wavelengths are used, an optical transmission module has to be independent of wavelength. The WDM-PON technology, satisfying the above condition, is being internationally studied and, from among various types of WDM-PONs, two types of WDM-PONs: a wavelength locking WDM-PON and a wavelength reuse WDM-PON, have been developed to a commercial level.
The wavelength locking WDM-PON uses a phenomenon that, if seed light from an external seed light source is injected into a particular Fabry Perot-laser diode (FP-LD), only light having a wavelength corresponding to that of the seed light is amplified and light having different wavelengths than that of the seed light are suppressed. In this case, a broadband light source (BLS) is used as the seed light source. In the wavelength locking WDM-PON, two types of BLSes are located in a base station. In more detail, one BLS provides seed light to an FP-LD included in an optical line termination (OLT) that is a base station optical device, and the other BLS provides seed light to an FP-LD included in an optical network unit (ONU) that is a subscriber side optical device.
A spectrum of light transmitted from a BLS is sliced while the light passes through a wavelength division multiplexing multiplexer (WDM MUX) included in the OLT and a WDM MUX included in a remote node (RN), and spectrum-sliced seed light is injected into an FP-LD. Meanwhile, the FP-LD has a strong polarization characteristic and thus seed light not having a polarization characteristic has to be used. In general, a spectral width of spectrum-sliced seed light is large and thus long distance transmission is restrictive due to dispersion of an optical signal.
The wavelength reuse WDM-PON uses a reflective semiconductor optical amplifier (RSOA) as a light source for communication. An optical signal having downstream data transmitted from the OLT loses the downstream data by an RSOA included in the ONU so as to be converted into continuous wave (CW) light. The CW light is modulated into upstream data so as to be transmitted to the OLT. Thus, a modulated optical signal transmitted from the OLT to the ONU functions as seed light in the RSOA included in the ONU. Meanwhile, an RSOA included in the OLT also requires seed light and, in general, an external light source is used. An RSOA device has a weak polarization characteristic and thus seed light having a strong polarization characteristic can be used. However, downstream data included in a modulated downstream optical signal cannot be completely removed by the ONU and thus a downstream optical signal component remaining in an upstream optical signal deteriorates a quality of upstream transmission.